Electrical starting of small aircraft engines has, in the past, been accomplished using dc starter-generators or separate dc starter motors. A few generally unsuccessful attempts have been made to use synchronous ac generators as starters by using a constant speed drive as a torque converter, between the ac generator and the engine. In these latter systems, the synchronous machine was brought up to near-synchronous speed, using the damper or amortisseur winding on the rotor as an induction-starter, and then the dc rotor-field was energized to lock the rotor in synchronously with the rotating ac field. From this point, the engine was brought up to speed, using the constant speed drive as a torque-transmission device between the synchronous-motor and the engine. More recently, because of the lack of success of starting large aircraft engines with conventional synchronous generators, pneumatic starters have been used in most wide-bodied jets.
One of the few exceptions to the pneumatic starting method has been the use of the recently developed permanent-magnet (samarium cobalt) generator as a starter. This starting system used a reverse-mode operation of a static power converter known as a "cycloconverter." In its generating mode, the system operated as a variable speed constant frequency power system in which the high frequency samarium cobalt generator power is converted to 400 Hz constant frequency/constant voltage power. One of the problems of such a system, however, was that the input frequency to the "cycloconverter" had to be at least 2 to 4 times higher than the output. Consequently, the generator had to run at very high speeds, (i.e., about 11,000 to 22,000 rpm) and utilize a large number of electromagnetic poles.
With the prospective emergence of the "All Electric Airplane" and the proposed use of large capacity generators, the subject of synchronous ac motor starting is again appropriate, since the generators are in a kilowatt range that makes them potentially suitable for starting even the very large aircraft engines. However, although these generators are likely to use permanent magnet type rotors, "straight" synchronous motor starting is not possible--particularly as there is no amortisseur winding. In answer to these problems, the present invention provides a unique and novel method and arrangement for integrating an induction motor within a permanent magnet type rotor in order to start turbine aircraft engines.
Use of a starter-generator attached to a single engine pad of an aircraft turbine is disclosed in U.S. Pat. No. 3,577,002 to Hall. The patent describes the use of a modern brushless ac generator operating in the role of a generator starter, but discloses use of a commutator and brushes rather than a completely brushless starter-generator configuration in accordance with the present invention. To offset the problem of operating the commutator and brushes after the engine is started, an electrically activated means is provided to lift the brushes off the commutator. Use of such a dc motor, however, is unsuited to starting of large aircraft engines, as for example 50,000 to 60,000 lb thrust engines.
Another patent which teaches utilization of a generator to start an aircraft engine is U.S. Pat. No. 3,264,482 to Clark et al. The '482 patent discloses a different system from the present invention which integrates a permanent magnet generator within a gas turbine engine. The permanent magnet generator performs the dual functions of power generation and synchronous motor starting. In performing the starter function, the generator follows the aforementioned method of using "power-electronics" to supply the starter-generator with variable voltage/variable frequency power. The present invention is designed as an advance over the '482 system which does not include an induction rotor and does not use any supplementary means to accelerate a rotor up to a "synchronization point."
A machine which includes a synchronous ac machine and a permanent magnet machine within a single housing is disclosed in U.S. Pat. No. 4,139,780 to Hucker et al. The machine, known as an integrated drive generator, is based upon a widely used practice of integrating a constant speed drive with a conventional wound-rotor generator and combines a synchronous ac primary generator/exciter/permanent magnet machine with a conventional hydromechanical constant speed drive. Unlike the present invention, this machine has as its primary objective, the generation of constant-frequency power; also it is not directed to aircraft turbine engine starting. As such, it does not utilize an induction rotor but rather describes the permanent magnet generator merely as a source of "control-power" for the voltage regulator/supervisory power, etc.
By contrast, British specification No. 820,180 to Harbord describes a co-axial integration of an induction machine within an inductor alternator. The machine, unlike the present invention, is not practically implementable for large aircraft engine starting. While the '280 machine utilizes an induction rotor (10) attached to a rotatable hub (13) attached to the squirrel cage induction rotor (21), the present invention describes the utilization of a high magnet strength permanent magnet rotor and a novel mechanical implementation of an induction motor/gearbox/clutch cartridge inside the permanent magnet rotor structure. Thus, the induction motor of the present invention is used novelly with a modern electronic start method.
From the foregoing it can be seen then that it is a primary object of this invention to provide a novel aircraft engine starting system which utilizes a synchronous generator as a starter of large and small aircraft engines by overcoming the problem of the "non-self-start" characteristic of a synchronous motor.
A further object of this invention is to provide a machine for starting large turbine aircraft engines which utilizes a highly reliable and robust induction motor to furnish the self-start portion of the engine start cycle.
It is also a further object of the present invention to provide a machine for starting aircraft engines which combines an induction motor with a synchronous permanent magnet type generator for carrying out the engine start function, and to provide at least two processes for implementing this function.